In the following subsections, we present additional model reduction approaches based on Krylov subspace projection methods: refer to [1]} for a textbook. Notice that \({\mathbf {A}}\in \mathbb {R}^{N\times N}\)  will stand for \({\mathbf {A}}^I\) , i.e. the scaling matrix \(\mathbf {B}\)  defined in previous section for the sake of generality will always be equal to the identity to restrict our investigation.
